Method of preparing lead cyanamide



SePt- 3, l940- K. D. ASHLEY Er AL 2,213,441

METHOD 0F PREPARING LEAD CYANAMIDE 3 Sheets-Sheet 1 Original Filed Feb. 2, 1939 sept. 3, 1940.

Original Filed Feb. 2, 1939 K, uAsl-ILEY` Er AL 2,213,441

METHOD OF PREPARING LEAD CYANAMIDE 5 Sheets-Sheet 2 @P005 Ca CNE INVENTORS /fr/wvfzw o. ,46x/.4 fr,

elf/PM 5. C24/Pfr, BY x g ufl-.1 M

ATTORNEY.

Sept. 3, 1940.

K. D. ASHLEY ET AL METHOD 0F PREPARING LEAD CYANAMIDE oiginal Filed Feb. 2, 1939 ICAUDE C0 CVV@ CHL C/UM 3 Sheets-Sheet 3 A proven to be much superior in light fastness to nated at some stage of the manufacturing proc- 10 are much more stable under these conditions. by adding soda ash prior to precipitation of the 1 5 Patented sept. 3, 1940 2,213,441

UNITED STATES PATENT OFFICE METHOD 0F PREPARING LEAD CYANAMIDE Kenneth D. Ashley, Darien, Conn., and Cyril B. Clark, Scarsdale, N. Y., assignors to American Cyanamid Company, New York, N. Y., a corporation of Maine Original application February 2, 1939, Serial No. 254,200. Divided and this kapplication April 18, 1940, ASerial No. 330,234

2 Claims.v (C1. 523-78) l The present invention relates to methods of under these circumstances, the lead cyanamide is .manufacturing lead cyanamide. thrown down in a physical form which not only The Value of leadcyanamide as a pigment is facilitates the filtering and washing thereof but well known in that it has a covering power equal the desirable characteristics of the final product 5 or superior to primrose yellow, which is an are thereby enhanced.

accepted standard in this trade. However, the Inasmuch as calcium compounds in the final chrome yellows have a tendency, vunder the inproduct tend to decrease the covering power of uence of sunlight, to fade and lose their color, the yellow pigment, it is desirable, where this is .whereas a good grade of lead cyanamide has been important, that such compounds should be elimi- K that of any `other similar material tested. ess.

It is well known that chrome yellows are not The invention contemplates alternate methods particularly alkali resistant, whereas it has been for the removal of calcium from the system. The found that pigments containing lead cyanamide preferred procedure is to throw out the calcium Another valuable property inherent in lead cylead cyanamide. A modified procedure contemanamide is its usefulness as an ingredient of plates the precipitation of calcium as a sulfat'eby under-water paint in that it has a tendency to adding sulfuric acid to the water extract of the prevent the adherence of marine growths on crude cyanamide starting material followed by surfaces painted therewith. In such cases, the the fOIIlatOIl 0f lead yanamde. In 2i third 20 particular color or purity of vshade is not of modification, the calcium may be removed from importance and, consequently, poorer grades may the system after the formation of lead cyanamide be used for this purpose without detracting from by adding to a slurry of the crude materialacidc its usefulness. substances, while maintaining the slurry alkaline The literature proposes the manufacture of in reaction. This'has the effect of converting the 25 lead cyanamide from commercial calcium cyanacalcium compounds to soluble or relativelysoluble mide by treating a sulfur-free water extract salts which may be removed in a filtering or thereof with a solution of a lead salt. When, washing operation. however, such procedures were followed, it has The invention further contemplatesthe novel 3o been found that mechanical manipulation of the steps and combination of steps hereinafter more 30 thus produced lead cyanamide is difficult, due, particularly described.

perhaps, to the physical form in which the com- The drawings show a series of ow sheets illuspound is thrown out of solution and, moreover, trating alternative methods of .carrying out the the nal product is inferior in quality. processes of the invention.

The principal object of the present invention, Referring now to Fig; 1, which is the preferred 35 therefore, is the production of lead cyanamide method of carrying out the invention, we desire which may be made without manufacturing diflito use as a starting material a fully hydrated culties and from reasonably cheap sources of crude calcium cyanamide. This cheap, convenmaterial. ient, commercial product contains approximately 40 Where such substances are to be used as pig- 20% cyanamide nitrogen together with impuri- 40 ments and it is desired to take advantage of its ties such as calcium hydlGXde, graphite, Combrightness of shade, covering power and/or light bned sulfur and other solid impurities. Inasfastness, it maybe prepared according to the much as any sulfur present during the formation present methods as high as 96% pure, which is of the lead cyanamide is converted to lead sulfide, of distinct value. it is desirable to remove such sulfur prior to that 45 To this end, the invention contemplates in one step in order to maintain the bright color of the of its broadest aspects, the manufacture of lead lead cyanamide, where it is to be used as a pigcyanamide by treatment of a water slurry of ment. This sulfur may be readily removed by crude calcium cyanamide to remove sulfur and slurrying the crude calcium cyanamide in water other objectionable compounds, followed by preand adding thereto requisite quantities of a lead 5 cipitation of lead cyanamide from a solution of a `compound such as lead carbonate, basic lead cyanamide by means of yellow lead oxide prefercarbonate .or lead oxide. Under these circumably relatively pure, such as is obtained by substances, no undesirable reaction products are linliming litharge, or by means of lead carbonate or troduced. Upon passing this magma to a sepathe basic carbonate. It has been found that vration equipment, such as a filter, the solids, ge'n- 55 erally consisting of the lead sulfide, calcium hydroxide, graphite, silica and other solid impurities are removed. The ltrate from this operation consists of a solution of acid calcium cyanamide. Where color is of no importance, however, this step may be omitted, although it will be desirable to filter the slurry to get rid of the other solid impurities.

In order to remove calcium from the system, a suiiicient quantity of soda ash may now be added to precipitate the calcium as carbonate which at the same time converts the cyanamide to acid sodium cyanamide.

The solution of sodium acid cyanamidemay now be readily converted to lead cyanamide by adding thereto yellow lead oxide, lead carbonate or basic lead carbonate. A reasonably pure grade of yellow lead oxide is preferred inasmuch as this gives a final product of good color without contaminating impurities. Where color is not of particular importance, such for instance as where the lead cyanamide is to be used in an under-water paint, ordinary litharge may be used. However, due to the fact that ordinary ltharge contains some quantities of red oxide of lead and this substance appears to be unreactive with the acid sodium cyanamide, it appears in the final product as such, with consequent dulling of the color. As above stated, however, this may not be of any moment in some circumstances.

Due to the fact that the acid sodium cyanamide solution is distinctly alkaline, the reaction with yellow lead oxide or carbonate proceeds with amazing smoothness and the lead cyanamide resulting therefrom is in such physical form as to make its separation from the solution in which it is formed comparatively simple. This is not true if lead nitrate or other soluble lead salts are used at this point. The resulting lead cyanamide may be readily washed and'dried to a substantially pure product.

In Fig. 2, an alternative procedure is shown which is substantially identical in all respects to that above described down to the formation of the solution of acid calcium cyanamide. According to the Inodied procedure, yellow lead oxide, preferably such as is obtained by subliming litharge, may be added at this point with the formation of lead cyanamide as before. However, this magma also contains all of the calcium which must now be removed if a product of good covering power is desired. These calcium compounds may be substantially eliminated by leaching the solid product of the lead oxide or carbonate reaction step with an acidic material such as hydrochloric, nitric, acetic or sulfuric acids as rather dilute solutions and added slowly so as to maintain the reaction of the slurry alkaline. In this operation, the calcium is converted to a soluble salt which may be readily washed from the lead cyanamide, leaving that product in a relative state of purity.

In the modification shown in Fig. 3, the procedure above described in connection with Figs. l and 2 is followed down to the formation of acid calcium cyanamide. If now requisite quantities of sulfuric acid are added thereto, the calcium content will be thrown down as the sulfate which may be removed by filtration, leaving free cyanamide in the filtrate. Soda ash or caustic soda may now be added to the iiltrate until all of the free cyanamide therein has been converted to acid sodium cyanamide or nearly so. The addition of yellow lead oxide to this solution under the conditions as above described for the second amide.

Example I 100 parts of completely hydrated calcium cyanamide and 570 parts of water are stirred together for approximately minutes at room temperature. A paste is made by adding 50 parts of water to 100 parts of yellow lead oxide, lead carbonate or basic carbonate with agitation until smooth and all the small lumps are broken up. 7.5 parts of this lead paste now thinned with a small quantity of water are added to the cyanamide slurry with agitation to remove sulfur compounds present in the latter. The solution is now filtered and the cake washed with water, the washings being added to the main solution.

Soda ash is now added to the clear solution of calcium acid cyanamide to precipitate the calcium therefrom, approximately 12'7 parts of a 21% solution being required. An excess of sodium carbonate should be avoided at this point because otherwise basic lead carbonate forms when the lead cyanamide is precipitated in a later step. The precipitated calcium carbonate is then ltered and the cake washed.

The remainder of the lead paste, thinned with a small amount of water, is then added to the cyanamide solution with good agitation. The lead cyanamide thus formed is permitted to settle, the mother liquor decanted and the product thoroughly washed. The lead cyanamide may then be dried at from 70 to 75 C.

Example II A solution of calcium acid cyanamide is prepared according to the method of Example I. To this solution add a suiiicient quantity of the lead paste, to precipitate requisite quantities of the lead content as cyanamide. The magma is then ltered and the cake washed alternately with 2% hydrochloric acid and Water until the washings show substantial freedom from calcium. The cake should then be washed with water until free of chlorides and dried as in Example I.

Ezcample III Where color or covering power is of no moment, the calcium extraction step may be omitted as Linder these circumstances, the presence of the calcium compounds is not particularly detrimental.

Example IV Prepare a iiltered solution of calcium acid cyanamide as in Example I and add thereto approximately 170 parts of a 20% solution of sulfuric acid to neutralize the solution and precipitate the calcium as sulfate. The latter may be removed by filtration and the cake washed. To the clear solution, 67 parts of 40% alkali metal hydroxide solution such as sodium hydroxide is added to convert the cyanamide to acid sodium cyanamide. To this solution, the lead paste may be added and the procedure set out in Example I for the recovery of lead cyanamide followed.

It has been determined that during the precipitation of lead cyanamide, it is desirable that a condition of strong alkalinity be maintained if a good colored product is to be obtained. The procedures recommended above insure this desirable result. It has also been determined that Iit is important to maintain an excess of cyanamide solution during the lead paste addition, that is, a quantity of lead not exceeding that sufiicient to combine with '76% of the cyanamide content of the solution should be added. Under these conditions, a 95% product can be obtained. On the contrary, where the excess of cyanamide dropped to as low as 11%, the purity of the lead cyanamide product likewise decreased.

Even in the better grades of lead cyanamide produced according to these methods, it may contain as much as of unreacted lead compound. However, inasmuch as such oxide is of a yellow shade, and the carbonate is white, it is not particularly detrimental to the normal uses to which a lead cyanamide pigment may be put. This is especially true where a good grade of yellow lead oxide is used as the starting material. Where, for instance, a crude grade of litharge constitutes one of the reactants, its content of red lead oxide remains unreacted and has a tendency to contaminate the color in the final product.

Samples of a good grade of lead cyanamide broadly and restricted only by the scope of the l claims.

We claim:

l. A method of producing lead cyanamide which includes the steps of slurrying calcium cyanamide in water to form a solution of calcium acid cyanamide, adding a compound chosen from the group consisting of yellow lead oxide, lead carbonate and basic lead carbonate thereto, treating the magma resulting therefrom and containing lead cyanamide and calcium compounds with an acid to convert the latter to soluble salts, leaching the same therefrom and recovering the lead cyanamide.

2. The method of claim 1 in which the magma is maintained alkaline in reaction during the acid additions.

KENNETH D. ASHLEY. CYRIL B. CLARK. 

